1. Technical Field of the Invention
The present invention relates to an alternator, and in particular, to an alternator to be loaded on a passenger car or a truck, for example.
2. Related Art
An alternator for a vehicle (hereinafter referred to just as an “alternator”) is loaded on an engine to charge a battery or supply current to on-vehicle machinery which imposes electrical loading (hereinafter referred to as a “current consumer”).
For example, as described in Japanese Patent Application Laid-Open No. 3-060338, an alternator generally includes a rotor, an excitation winding, a stator, a rectifier and a voltage regulator, which are all integrally constructed (refer to pp., 2 to 6 and FIGS. 1 to 6).
The rotor has field poles which are magnetized by the excitation winding. The stator has an armature winding which generates electric power by receiving magnetic flux from the field poles. The rectifier full-wave rectifies the AC output that has been generated by the armature winding for conversion into DC. The output voltage produced in this way is then regulated by the voltage regulator. Thus, the alternator outputs DC current with regulated output voltage.
Electric power control in such an alternator is performed by detecting battery voltage and increasing/decreasing current to be supplied to the excitation winding through a brush for adjustment of the amount of magnetic flux in the magnetic field.
Electronics devices for improving safety and comfortable driving have recently come to be loaded on vehicles, and this tends to increase electrical power consumption in vehicles. On the other hand, since more importance is placed on environmental protection or natural resources saving, vehicles are now required to provide more improved fuel consumption. In order to improve fuel consumption, low idling of engine has been pursued further, or reduction in size and weight has been pursued for the apparatuses to be loaded on vehicles, including the alternator mentioned above. These measures for improving fuel consumption, however, also require suitable electronics devices, so that the tendency of increasing electrical power consumption has been reinforced. In this way, sufficient electric power generation is unlikely to be achieved in vehicles currently.
Under the circumstances as mentioned above, it has been required that alternators should more increasingly generate electric power than ever in a low-speed region of engine revolution (“a low-speed region of engine revolution” hereinafter abbreviated as “a low-speed region”), where power-generating performance is low, to afford electric power to be supplied to the electronics devices.
However, in an alternator, in general, electric current equal to or more than a current value determined by battery charge voltage cannot be supplied to the excitation winding. Therefore, it is not easy to increase a maximum output current. One approach may be to enlarge the size of the alternator and supply that much more electric power. This approach, however, may increase the output current, while deteriorating feasibility of loading the alternator on a vehicle and impeding reduction in the weight of the vehicle, and thus may raise problems if put into practice.
Another known technique for increasing electric power generation without deteriorating the feasibility of loading an alternator is suggested in Japanese Patent Laid-Open No. 6-225473, for example, in which current supplied to an excitation winding is supplied by a separately provided power source external to the alternator (refer to pp., 3 to 7 and FIGS. 1 to 4). Use of a separately provided power source may enable increase of voltage to be applied to the excitation winding to thereby increase the electric power generation by that much.
In the technique disclosed in Japanese Patent Laid-Open No. 6-225473, the separately provided power source external to the alternator has a configuration in which a wiring is drawn from the armature winding of the alternator to take out three-phase AC voltage, which voltage is then boosted by a three-phase transformer and rectified, and this rectified voltage is applied to an excitation winding through the wiring again. The configuration is also provided with a switch for switching between self excitation and separate excitation. This configuration has a problem that various additional parts, such as a wiring, a transformer, a rectifier and a switch, are required. Further, the separately provided power source which is used only for exciting the alternator is so large that it constitutes a factor for impeding weight reduction of a vehicle.
In addition, the voltage induced in the armature winding of the alternator is in proportion to the number of revolutions of a magnetized rotor. Thus, when electric power is generated in a high-speed region of engine revolution (“a high-speed region of engine revolution” hereinafter abbreviated as “a high-speed region”) by utilizing this configuration, the voltage applied to the excitation winding (i.e. the voltage boosted and rectified by the three-phase transformer) becomes so large that, although the current flowing through the excitation winding is increased, heat built up in the excitation winding itself is also problematically increased. As a result, deterioration is accelerated in the insulating film of the copper wire constructing the excitation winding to resultantly shorten the life of the insulating film, making it difficult to provide an operation which is stable for a long period of time.
A separate-excitation type configuration in which one end of the excitation winding of an alternator is connected to an externally provided separate power source may be realized by connecting the alternator to a battery directly or through a DC-DC converter.
In case electric power is supplied to the alternator through the DC-DC converter, such additional parts as the DC-DC converter, a wiring and a switch are required. Moreover, although the DC-DC converter has a large size and contributes to increasing the amount of power generation, it does not satisfy the requirements of reduced size and weight of a vehicle.
In case electric power is directly supplied from the battery to the alternator, the connection between the battery and the excitation winding of the alternator is made, for example, via a switch and a wiring. Therefore, the voltage decreases due to the wiring resistance, for example. Comparing with a self-excitation type configuration, the separate-excitation type configuration reduces current that can be supplied to the excitation wiring, whereby the output current of the alternator is problematically reduced.